A requirement exists to provide a simple, low cost means for interconnecting equipment such as computers, imaging apparatus, and consumer electronic devices. A serial data bus configuration has been standarded by the IEEE and is designated as 1394-1995. The 1394 standard multiplexes a variety of compressed digital audio and video signals, MIDI and device control commands onto two twisted pair conductors and provides for connections to 63 peripheral nodes with data transmission rates of up to 400 Mbps. Devices are interconnected in a daisy chain manner by a small, thin, flexible cable comprising three twisted pairs, terminated with standardized, rugged connectors capable of multiple insertions. The three twisted pairs are assigned as follows, a first conductor pair provides data transmission/reception, a second conductor pair are used for data strobe signaling. The third conductor pair provides one conductor as a power supply ground with the second conductor providing an optional coupling for power insertion or extraction.
Data transmission and reception is facilitated utilizing the well known data coding method known as non-return to zero, or NRZ. Data transmission is accompanied by a strobe signal which changes state whenever two consecutive NRZ data bits are the same. Thus the data and strobe signals may be processed by an exclusive OR function to form a clock signal which marks transitions in either data or strobe signals. This method of data and strobe signal coding is described in U.S. Pat. No. 5,341,371.
The third conductor pair of the IEEE 1394 standard provides an optional power capability. This powering option may be ignored, utilized to supply an unregulated DC source to the bus, or may provide power to energize a bus node. The unregulated power supply voltage may have a value between 8 and 40 volts and is required to supply a maximum of one ampere per connected node without the supply voltage falling below 8 volts.
In a simplified digital data communication system, for example an audio video interface system, data transmission and reception may be facilitated by the method employed in the 1394 data bus standard. However, simplification of the interface system may be achieved if only a single data source is permitted to occupy any one data bus segment. This simplification eliminates the need for much of the digital circuit complexity associated with a link layer required by the standard, and analog signaling associated with a physical layer of the standard. In such a simplified system, control could be achieved by means of a single conductor, wired-OR control bus. In addition a simplified interface system may beneficially utilize the standardized cable and connectors. However, such use of the standardized cable and connectors as a simplified bus could permit an undesirable connection between the simplified bus apparatus and equipment conforming to the standard. Thus, beneficial utilization of the standardized cable and connectors by the simplified system described above is precluded by the risk of consequential circuitry damage resulting from such non-compliant use, inconsistent with the standard.